1. Field of the Invention
The present invention relates to an image reading device, such as an image sensor, that converts optical information to an electric signal.
2. Description of the Prior Art
As FIG. 3 shows, an image reading device is composed of IC chips K1, K2, . . . , Km, for reading an image, arranged in a row and mounted on a printed circuit board (not shown). The IC chips K1, K2, . . . , Km sequentially output photoelectric conversion signals, which are then converted by an A/D converter 100 into a digital signal, which is then fed out via an output terminal 200. The IC chips K1, K2, . . . , Km all have the same circuit configuration, of which a conventional example will be described below with reference to FIG. 4.
Photodiodes PD1, PD2, . . . , PDn, serving as photoelectric conversion elements, have their anodes connected to ground, and have their cathodes connected respectively to the gates of p-channel MOSFETs (metal-oxide semiconductor field-effect transistors) A1, A2, . . . , An for amplification. The transistors A1, A2, . . . , An receive, at their sources, constant currents respectively from constant-current sources I1, I2, . . . , In, and have their drains connected to ground.
A bias voltage supply circuit 2 outputs a positive direct-current voltage stably. The voltage (hereinafter referred to as the “bias voltage”) output from the bias voltage supply circuit 2 is applied, through the drain-source channels of p-channel MOSFETs B1, B2, . . . Bn for switching, to the cathodes of the photodiodes PD1, PD2, . . . , PDn respectively.
In this circuit configuration, when the transistor Bx(x=1, 2, . . . , n) remains on for a predetermined time or longer, a reverse bias is applied to the photodiode PDx, and thus a predetermined amount of electric charge is accumulated therein. In the following descriptions, this will be depicted as “the photodiode PDx being initialized.”
When the transistor Bx turns from on to off, the initialization of the photodiode PDx is broken off, and the accumulated electric charge is discharged in proportion to the amount of incident light, causing a current to flow. Thus, the photodiode PDx gives a voltage proportional to the amount of incident light.
Moreover, a transistor Cx turns on, with the result that the voltage signal (hereinafter referred to simply as “the signal of the photodiode PDx”) at the cathode of the photodiode PDx is amplified by a source-follower circuit formed by the transistor Ax, is then subjected to further amplification, waveform shaping, and other processing by an output circuit 51, and is then fed out via a terminal TOUT (hereinafter, this will be depicted as “the signal of the photodiode PDx being read out”).
A control circuit 52 controls the transistors Cx in such a way that the signals of the photodiodes PD1, PD2, . . . , PDn are output sequentially, and also controls the transistors Bx in such a way that the photodiode PDx is initialized every time the signal of this photodiode PD1 is read out.
However, in this conventional circuit configuration, the signals of all the photodiodes PD1, PD2, . . . , PDn are transmitted to the output circuit 51 by way of a single output line L, which is thus provided so as to be common to as many as several tens to several hundred photodiodes. This makes the output line L considerably long, and thus imposes a limit on the operating frequency of the image reading device, because, the longer the conductor laid as the output line L, the higher its resistance and capacitance, and thus the greater the resulting time constant.
Moreover, in the conventional circuit configuration, there are variations in the characteristics of the individual photodiodes PD1, PD2, . . . , PDn, but nevertheless the signals read out from them are fed out intact. As a result, according to the position of the photodiodes within the IC chip, for example, the characteristics of the semiconductors, such as their degree of impurity and thickness, and the light shield conditions for the individual photodiodes vary, and thus the signals that these photodiodes output vary.